A spectacle lens having a refractive power portion in which the refractive power changes progressively is known. For example, a distance-near progressive power lens is designed such that the dioptric power changes progressively on a principal meridian so that a wearer can see an object clearly and seamlessly from a long distance to a short distance. Many of spectacle lenses of this type are designed depending on prescribed individual dioptric powers for left and right eyes and a wearing condition; however, for a case where a difference exists between prescribed distance dioptric powers for left and right eyes, such as anisometropia, conventional lens design was not suitable. The term anisometropia as used herein means a case where a difference exists between dioptric powers of left and right eyes regardless of the magnitude of the difference.
For example, when a wearer of anisometropia performs binocular vision for a target positioned on a side in a state where the wearer wears spectacle lenses of which left and right distance dioptric powers are different from each other, the wearer is forced to perform unnatural convergence or divergence not accompanied by tonic accommodation or relaxation of accommodation so as to cancel a shift between the left and right visual lines caused by a difference between prismatic effects of the left and right lenses. Furthermore, the convergence and the divergence of this type changes a position on a lens through which the visual line passes from a position assumed in design, which deteriorates the aberrations for the both eyes and thereby hampers suitable binocular vision.
In view of the above, regarding a pair of progressive power lenses having left and right dioptric powers different from each other, U.S. Pat. No. 8,162,478 (hereafter, referred to as patent document 1) suggests a pair of progressive power lenses configured to ensure suitable binocular vision. Specifically, patent document 1 describes technology where a lens component of a pair of progressive power lenses having left and right distance dioptric powers different from each other is divided into a component for a pair of progressive power lenses having the same distance dioptric power and the addition power and a component for a pair of single focal lenses having left and right dioptric powers different from each other, a ratio of moving amounts of visual lines on the lenses of the left and right eyes when an wearer moves the wearer's visual lines from a front far point to a far point other than the front while being oriented toward a predetermined azimuth angle in the state of performing binocular vision wearing the lenses having the component for the single focal lenses is calculated, and occurrence of aberrations other than the difference between the left and right distance dioptric powers is suppressed, in regard to the difference in the average dioptric power and the astigmatism between the left and right visual lines in binocular vision, by applying correction according to the ratio with respect to the average power distribution and the astigmatism of the lens component for a single eye or both eyes of the lenses having the component for the progressive power lens.